US3291655A - Alloys - Google Patents

Description

United States Patent 3,291,655 ALLOYS Robert F. Gill, Schenectady, Louis D. Tote, Ballston Lake, and Jack Keverian, Schenectady, N.Y., assignors g? (general Electric Company, a corporation of New or No Drawing. Filed June 17, 1964, Ser. No. 375,921 Claims. (Cl. 148-36) This invention relates to high temperature, high strength resistant ferritic steels which are characterized by improved creep-rupture ductility. More particularly, the invention relates to such alloys of the chromium-molybdenum-vanadium class which are characterized by good rupture ductility and high creep-rupture strength.

The use of Cr-Mo-V steels for high temperature, high stress service is well known. However, it has been found that when such steels are heat-treated at high austenitizing temperatures to increase the creep-rupture strength, the creep-rupture ductility tends to be lowered. It will be apparent that it is desirable not only to increase the creeprupture strength of such steels, but to maintain the rupture ductility or resistance to embrittlement at optimum values. Where optimum creep-rupture strength is desired in Cr-Mo-V steels, complete solutioning of the vanadium carbides is required followed by cooling rapidly enough to allow the steel to transform to a predominantly bainitic microstructure followed by a tempering treatment usually in the temperature range of from about 1150 F. to 1400 F. to precipitate the vanadium in the form of vanadium carbide in a fine, uniformly dispersed phase. In this form, vanadium carbide is an eflicient strengthener for high temperature creep and the finely dispersed vanadium carbides result in high hardness and creep-rupture strength, but at the same time a higher incidence of creep-rupture embrittlement. It has been common practice in the use of Cr-Mo-V steels, where creep-rupture ductility is important, to use a lower austenitizing temperature than that which is required for complete solution of the vanadium carbide. However, this reduces the creep-rupture strength appreciably and detracts from the use of the alloy in high temperature service where high stresses are encountered.

From the above it will be quite apparent that there is a need for Cr-Mo-V steels which can be heat-treated to achieve not only high rupture ductility but high creeprupture strengths which will enable such alloys to resist high temperature stresses. Accordingly, a primary object of the invention is to achieve in high temperature Cr-Mo-V steels high rupture ductility and high creep-rupture strength at one and the same time.

Briefly, the invention relates to increasing the high temperature creep-rupture ductility at high creep-rupture strengths of cast and wrought ferritic steels of the Cr- Mo-V class by the addition to the molten alloy of aluminum and titanium. It has been found that in an alloy of the above class comprising by weight about 0.05 to 0.6% carbon, .5 to 3% chromium, 0.3 to 1.75% molybdenum, 0.15 to 1% vanadium, 0.2 to 1.5% manganese,

with the remainder essentially iron, the rupture ductility of the alloy at high creep-rupture strengths is maintained by the addition to the molten alloy of such quantities of aluminum and titanium that at least about 0.02% aluminum and at least about 0.04% of titanium based on the weight of the molten alloy are present in the final alloy, the remainder of the aluminum and titanium added being in the combined state. Up to .2% residual aluminum and titanium have been found useful. The alloys of the invention are austenitized at such temperatures generally over 1800 Fl that essentially all of the vanadium carbides are dissolved. The alloy is then cooled to allow transformation to a predominantly bainitic structure and tempered as above.

The Cr-Mo-V steel alloys of the present invention are the result of a balance of ingredients or constituents which combine to provide the improved characteristics obtained. The carbon content of the alloy should be held to .05 to .6% if the optimum combination of high temperature characteristics is to be obtained. With carbon contents less or greater than those prescribed, it has been found that the high temperature creep-rupture strength of the steel is significantly lowered. Lower amounts of carbon within the above range are used where welda-bility is an important factor. The chromium content of the all-0y is likewise quite critical and should be held to between .5 and about 3%. Additionally, a minimum of about .5% is required to provide resistance to g'raphitization. Commonly, from about .75 to 1.5% provides for oxidation resistance and this amount may be increased to about 3% when maximum oxidation resistance is desired. More than about 3% of chromium on the whole results in an undesirable reduction in creep-rupture strengths. The molybdenum content of the alloy should be maintained between about 0.3 to 1.75%. Less than about 0.3% molybdenum results in poor creep-rupture strength while more than about 1.75 molybdenum promotes the undesirdable development of molybdenum carbides at the expense of vanadium carbide, again giving a lower creeprupture strength. Vanadium in amounts of from about 0.15 to 1% results in improved creep-rupture strength. The prescribed manganese content of about 0.2 to 0.5% insures at its lower level that sulfur is in the form of manganese sulfide and larger amounts of manganese enhance the hardenability of the alloy. While amounts of manganese over about 1% lead to some slight decrease in creep-rupture strength, this is not significant in amounts up to about 1.5%.

Listed in Table I below are the percent by weight compositions of a number of alloys made in connection with the present invention. Shown in Table II are the room temperature, the tensile strength, yield strength, percent elongation and reduction in area under physical testing.

In Table I the percentages of aluminum and titanium in parentheses are those amounts added to the molten alloy, the other stated amounts of these metals being those present in the final alloy.

Alloys 1 through 21, 30 through 35 and 41 were prepared in an induction furnace. Alloys 22 through 24 and 36 through 38 were made in an electric furnace while alloys 26 through 29 were made in a plasma arc furnace. These varying modes of preparation account in part for the varying amounts of residual aluminum and titanium compared to the amounts added.

TABLE I Alloy No. C Mn Cr Mo V Ti Al .24 62 1. 28 1.03 067 l 013 24 61 1. l9 97 68 063 l 018 31 .74 1.26 1.12 .26 .()1 .042 31 52 1. l0 1. 10 23 061 008 TABLE 'I'Continued 1275 to 1325 F. for 20 hours. Alloys 26 through 29 were heat-treated for 12 hours at 1875 to 1925 F., air- Alloy No. Mn Cr M0 V Ti Al cooled and heatatreated at 1300" F. for 25 hours. Alloys 30 through 35 were heat-treated for 15 hours at 1925 27 1103 gag F., furnace-cooled, further heat-treated for 15 hours at 28 J56 L24 (1 13-64; F. and air-coiolfed. 12Alloys 36 tl11ggufih138 wgrehall initia ly heat-treate or ours at an en .15 40 29 12 1 25 96 46 8 furnace-cooled. Alloy 36 was further treated for 20 hours 30 &3 mm at 1350 F. and air-cooled, alloy 37 was further heat- 31 .16 .60 1.32 .93 .56 .110 .024 10 treated at 1350 F. for 7 hours, while alloy 38 was further 3 16 64 1 30 99 57 3% 2? heat-treated at 135 0 F. for '8 hours. Cast alloy 41 was k. 19 K. 05) heat-treated for 16 hours at 1875 to 1925 F., air-cooled, 33 J53 $98 fig heat-treated at 13 64 F. for hours and air-cooled. 34 .16 67 1.35 .96 .60 11g) 2g Shown in Table HI below are the creep-tupture proper- 35 .17 .64 1.35 1.00 .56 '12 .013 15 of the above alloys- .21 .01) 36 22 63 1. 41 .93 .64 0g; 3g 37 21 55 1 4 1 0g 64 TABLE IIL-CREEP-RUPTURE PROPERTIES 1 38 19 65 40 O0 58 013 Alloy Temp., Stress Rupture Elongation, R.A., 41 .16 .59 1.30 1.00 .57 .19 .023 Life, hours percent percent Res1dual alummum none added 1 w 3;, gg 2g 2. 80 13 00 ,0 2.50 9.9 TABLE H 9 1,100 60,000 12.0 10.0 52 a 131 13 1 61 -0.0 7.3 Tenslle .27 Yleld Elongation RA. I Alloy No. Strength, 3050111 011, Pereent Percerlt 10 fig 31883 3: 2 E2 1, 100 46, 000 217. 0 3. 9 17 1, 100 34, 009 1, 237. 0 2. 9 14 1 110,300 91,750 21.0 53.6 11 igg .8 8% 2-2 g-g 9.. 137,00 120, 500 21. 0 02. 0 100 000 O 0 58 10- 133,400 124,000 21.0 62.0 11100 4,000 1 750- 30 11- 137, 200 127, 500 21. 0 03. 0 200 000 1 0 0 61 14. 136,400 120,500 19.0 53.0 14 1-100 601000 4 8 5 15. 135, 600 120,000 21. 0 62. 0 100 000 0 5 8 17. 130,000 117,500 16.0 40 0 1 100 '000 1 4540 3' 1 7'3 13. 133,600 113,300 14.0 33.0 11200 0 19- 141,200 124,000 14.0 37.0 15 60,000 1m 0 72 20. 135,100 110,300 13.0 50.0 11100 461000 881 0 m 21. 140,000 122,000 17.0 53.0 11100 421000 1 0 4 23 22. 111, 300 93, 500 19. 0 61. 3 11200 251000 1 65d 0 1 47 23--. 113,500 95,500 13.5 50.0 17 11100 521mm 0 2 9 9 24--- 113, 700 96, 500 19.5 51. 4 11100 000 0 7 5 26--- 101,250 7 500 21.9 64.5 1 100 '000 1 2930 24 3'5 27-.- 102,000 77,000 22.3 63.7 11200 251000 113010 1750 8 1 18 I100 ,000 72. 0 9, 4 3 29.-- 91,750 61,250 22.3 69.9 1 100 46 000 128 0 3 1 13 30.-. 103,700 91,250 13.5 43.4 11100 29,000 1 5850 7 2 31-.. 107, 600 90, 250 11.0 13. 3 11200 000 4 1081900 911000 a 19 1'100 521000 121.0 12.0 59 33--. 110, 000 93, 250 20. 5 56. 0 11100 46' 000 401 0 12 0 50 34. 103,000 36,750 10.5 13.4 1,100 34,000 1,58 M 39 35- 107,500 39,500 17.0 449 1 200 25 000 230 O 22 0 74 36. 101, 700 76, 500 22.0 60. 3 20 mo 000 0 5 15 37- 106,400 1 73,000 21.0 59.6 n 11100 461000 312 0 17 33. 104,250 1 61,000 24.0 64. 0 11100 000 1 0 5 12 41 105,000 33, 375 21. 5 61. 3 11200 51000 0 0 41 21 1, 100 52, 000 74. 0 16. 0 77 1 0.02% offset yield strength. 2, 1, 200 25, 000 152. 0 14. 0 31 Alloys 1, 22 through 38 and 41 were cast wh1le alloys 22 1,100 38,000 407-6 13-7 9 1, 100 35, 000 1, 410. 1 14. 2 76 9 through 21 and 41 were forged. Alloy l was heat- 1,100 33,000 3,3324 14,7 73 treated for 16 hours at a temperature of 1.875 to 1925 i ggg 888 g g-g 3% F., air-cooled, heat-treated for 15 hours at 1325 F. and 23 I 33: 50 30416 1 air-cooled. The forged materials of alloys 9 through 15 $33 333 23-? g-g 21 were heat-treated at 1900 F. for 4 hours, air-cooled, and 2 1,300 14,000 417.2 13,2 73 heat-treated at a temperature of 1300 F. for tunes rang- 4 M88 238 1 328-; 11 1.3 2; ing from about 6 hours to 8 hours. The forgings of al- 1,100 36,000 1,337.2 3.1 33 loys 17 through 21 were all heated for 4 hours at 1950 5 {$88 32 888 1133 2 g 53 F., slow air-cooled over a period of 8 hours and reheat- 26 1,300 13,000 403.4 10.5 54 treated for 20 hours at 1850 F. and again slow air-cooled 1: lg? ggg Egg; 2 over a period of 8 hours. Alloy 17 was additionally 27 13.3 55 heated for hours at 1200 F. as was alloy 19. Alloys 11100 321000 44511 g1; g3 18 and 19 after the above heat treatment were again re 5 1,138 2% 888 113.9 3.2 3: heated at 1200" F. for 52 hours. Alloy 21 was further 28 1:100 351000 5 41 heat-treated at 1200 F. for 40 hours. Cast alloy 22 was 138 88% 39 7 13.12 g9 heat-treated for 12 hours at 1922 to 1940 F., air-cooled 29 1:100 301000 6 1 1 and heated for 28 hours at 1319 to 1328 F. Cast alloy 1.100 27,000 50443 0.3 88 a 1, 25, 000 1, 030. 7 16. 0 33 23 was heat-treated 01. 14 hours at 1895 1:0 1940 F., 3J1.- 70 30 10 3 00 34 19,1 84 cooled and further treated for 20 hours at 1301 to 1319 13g 83g 133 g8 32 F. Alloy 24 was heat-treated for 20 hours at 1922 to 1:100 301000 1,0365 10:3 53 1931" F., air-cooled and further treated at 1310 to 1328 31 1,100 38,000 83 1, 100 35, 000 110. 0 26. 5 34 F. for 23 hours. Alloy 25 was heat-treated for 10 hours 1,100 32,000 371.1 14 1 so at 1875 to 1925 F., air-cooled and further treated at 1,100 1 8451 15.1 67

TABLE III.CREEP-RUPTURE PROPERTIES-Continued Alloy Temp., Stress Rupture Elongation, R.A.,

No. F. Life, hours percent percent It will be seen from the above data that the addition to alloys of the type described of aluminum and titanium produces finished products which are not only characterized by high creep-rupture strength but at the same time are possessed of very desirable rupture ductility properties. It will be seen from a consideration of the data for alloy 1 wherein only aluminum is added to the alloy that the rupture ductility as evidenced by the percent reduction in area under stress at elevated temperatures leaves much to be desired in a cast product. Inspection of the data relating to Example 17 shows the same to be true of forgings to which only aluminum is added. Referring to cast alloy 38, it will be seen that the addition of titanium to the essential exclusion of or with only very small amounts of aluminum below the range prescribed produces an alloy which again has a decreasing rupture ductility at elevated temperatures. Alloy 18 illustrates the same effect for a forged material. Alloy 24 illustrates a cast material in which, while both aluminum and titanium are present, the relatively low amount of aluminum below the presently prescribed limits produces a material which is lacking in desirable rupture ductility. Alloy 14 illustrates this same effect of a deficient amount of aluminum for a forged product.

Alloy 27 is illustrative of a cast alloy in which, while a substantial amount of aluminum is present, there is a deficiency of titanium, once again, resulting in an undesirable rupture ductility. Alloy 28 shows that an increase in the amount of aluminum, still keeping the titanium at a low level, produces no pronounced improvement.

Alloy 15 (which contains 0.0016 boron for hardenability) illustrates a material having desirable rupture ductility by reason of its content of aluminum and titanium. Alloys 19, 22 and 34 are further illustrative of suitable materials according to the present invention.

Alloys 2 6, 27, 2'8 and 29 represent compositions which were obtained by making progressive additions of aluminum and finally 0.15% titanium to the same base heat of steel, castings being poured off after each addition. It will be noted that the rupture ductility progressively decreases through alloy 28 as the amount of aluminum is increased, showing that aluminum alone does not produce high rupture ductility. However, the addition of 0.15% titanium in alloy 29 results in a steel that shows no loss in creep-rupture ductility at all.

Alloys 9, 10, 11, 14 and 15 illustrate the eifect of varying amounts of titanium and aluminum. Alloy 9 contains no deliberate additions of aluminum or titanium and shows low rupture ductility. Alloy contains additional titanium but still shows no significant change in ductility. However, the addition of the aluminum indicated in alloys 11 and produced, as will be seen, a significant increase in rupture ductility.

In a similar manner alloys 17, 18, 19 and 21 show the beneficial efiect of titanium and aluminum additions 6 as in alloy 21 as compared to alloy 17 with aluminum alone or alloy 18 with titanium alone as pointed out above.

The alloys listed in Table I cover materials which are useful for many purposes. Alloys 17 through 21, for example, represent compositions which with the prescribed heat treatments are suitable for heavy forgings such as steam or turbine rotor forgings for high temperature service. Alloys 9 through 15 represent compositions and heat treatments suitable for smaller forgings and mill products such as plate and bar such as are used in pressure vessel construction for high temperature service. The remainder of the alloys represents compositions and heat treatments suitable for the production of castings, forgings or mill products for high temperature service where lower carbon contents are desirable for welding purposes.

It has been found that the order in which the aluminum and titanium are added is not critical and suitable steels have been made by adding either the aluminum or titanium first and also by adding both simultaneously. However, the recovery of the elements added is diflicult to predict in some cases. However, it is believed that the most consistent melting practice results if the aluminum is added first or simultaneously with the titanium. 'It is also useful to combine the aluminum and titanium as a single pre-alloyed master alloy with suitable proportions of the critical ingredients indicated. The additions of aluminum and titanium should preferably be made after the oxidizing and refining proportion of the steelmaking practice and just prior to tapping if the additions are to be made in the furnace. The additions can also be made in the ladle during or after tapping and all of the above practices have been used with equal success.

It has been found that vacuum degassing produces in some partial degree the beneficial results obtained by adding the present materials so that with vacuum degassing lesser amounts of the additives are required. The present invention is intended to cover such practice.

There is provided then by the present invention means for realizing to the fullest extent the high creep-rupture strengths which are obtainable in Cr-Mo-V steels, at the same time providing such steels which have a desirably high rupture ductility.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. A high temperature alloy characterized by high rupture ductility and high creep-rupture strength and having a bainitic structure with finely dispersed precipitated vanadium carbide, said alloy consisting essentially of by weight about .05 to .6% carbon, .5 to 3.0% chromium, 0.3 to 1.75% molybdenum, 0.15 to 1.0% vanadium, 0.2 to 1.5% manganese, from about .02 up to about 0.2% aluminum and from about 0.04 up to about 0.2% of titanium, with the remainder essentially iron.

2. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon 24%, chromium 1.31%, molybdenum 1.00%, vanadium .76%, manganese .82%, aluminum .031%, and titanium .074%, Wit-h the remainder essentially iron.

3. Analloy as in claim 1 wherein the constituents consist essentially of by weight carbon 30%, chromium 1.16%, molybdenum 1.08%, vanadium 28%, manganese .70%, aluminum 035%, and titanium .05%, with the remainder essentially iron.

4. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon 31%, chromium 1.18%, molybdenum 1.08%, vanadium .60%, manganese .81%, aluminum .02/8%, and titanium .075%, with the remainder essentially iron.

5. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon .13%, chromium 1.30%, molybdenum 1.00%, vanadium .48%, manganese .76%, aluminum .027%, and titanium 063%, with the remainder essentially iron.

6. An alloy as in claim 1 wherein the constituents consist essentially of by Weight carbon .12%, chromium 1.25%, molybdenum 96%, vanadium .46%, manganese .66%, aluminum .040%, and titanium .15%, with the remainder essentially iron.

7. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon .16%, chromium 1.32%, molybdenum .98%, vanadium 56%, manganese .66%, aluminum .024%, and titanium .110%, with the remainder essentially iron.

8. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon .16%, chromium 1.30%, molybdenum 39%, vanadium 57%, manganese 64%, aluminum .054%, and titanium .100%, with the remainder essentially iron.

' 9. An alloy as in claim 1 wherein the constituents consist essentially of by weight carbon .17%, chromium 1.34%, molybdenum 97%, vanadium .57%, manganese 153%, aluminum 110%, and titanium 100%, with the remainder essentially iron.

10. An alloy as in claim 1 wherein the constituents consist essentially of by Weight carbon .21%, chromium 1.4 1%, molybdenum 1.00%, vanadium .64%, manganese 8 .65%, aluminum 025%, and titanium 090%, withthe remainder essentially iron.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Journal of The Iron and Steel Institute, vol. 187, 1957, relied on pp. 314-327.

Journal of The Iron and Steel Institute, vol. 188, 1958, relied on pp. 915.

HYLAND BIZOT, Primary Examiner.

DAVID L. RECK, C. N. LOVELL, Assistant Examiners.

Claims (2)

1. 0.2% OF TITANNIUM, WITH THE REMAINDDER ESSENTIALLY IRON.
2. 1. A HIGH TEMPERATURE ALLOY CHARACTERIZED BY HIGH RUPTURE DUCTILITY AND HIGH CREEP-RUPTURE STRENGTH AND HAVING A BAINITIC STRUCTURE WITH FINELY DISPERSED PRECIPITATED VANADIUM CARBIDE, SAID ALLOY CONSISTING ESSENTIALLY OF BY WEIGHT ABOUT .05 TO .6% CARBON, .5 TO 3.0% CHROMIUM, 0.3 TO 1.75% MOLYBDENUM, 0.15 TO 1.0% VANADIUM, 0.2 TO 1.5% MANGANESE, FROM ABOUT .02 UP TO ABOUT 0.2% ALUMINUM AND FROM ABOUT 0.04 UP TO ABOUT